The terms "ristretto, espresso, lungo," etc have meaning only if we all use them in the same way. I'm posting a graphic that suggests a way to do this. The chart is based on two concepts:
(1) Bean freshness, bean variety, type of portafilter, pump pressure, and several other factors influence the amount of crema produced. Therefore, the measured volume of a shot can vary significantly. The only quick, accurate way to measure the actual amount of espresso produced is by weighing it, NOT by measuring it volumetrically.
(2) "Brewing ratio" is the best way to specify what a "ristretto," "regular espresso," or "lungo" is. Brewing ratio is the ratio of dry coffee used to liquid beverage produced. For example, a 32 gram espresso shot prepared with 16 grams of coffee would have a brewing ratio of 16/32 or 50%. Previously I called this the "extraction ratio," but I believe "brewing ratio" is the more descriptive term. It is already used by the SCAA in defining brewed coffee parameters.
When first introduced to this approach, many people feel the geek coefficient is too high. They feel more comfortable remaining confused and deluded, gliby mouthing the terms "ristretto," "espresso" and "lungo." But I think that many serious home and professional baristas are motivated enough to seek a better way to communicate.
This chart is a work in progress. The key factors are the ones highlighted in green. Your comments about what you consider a ristretto or regular espresso to be (measured in this way) are welcome.
(Click above to see larger image)